The present invention is generally related to data communication, and in particular to the evaluation of capacity for radio networks based on orthogonal frequency division multiplexing (OFDM) with adaptive modulation and coding (AMC).
In order to evaluate existing networks or to plan network structures for a radio communication network, it is necessary to determine the capacity and coverage of a network or part of the network and the expected load occurring for the network. Such an evaluation may be desired for setting antenna parameters, for modifying and adapting elements of a network and/or for constructing a new radio network. The capacity describes the potential throughput of data in the communication network, and with this the number of users which may be served by the network. Usually, a radio communication network will comprise several base stations or antenna sites, each covering a certain area. Users of the radio network located in these coverage areas will be connected to the network through at least one of the serving antennas of that area. Therefore, the site locations and parameters/settings of antennas have a large effect on the capacity and general efficiency of a network. In order to optimize such parameters of a network, various issues such as the number of users and their location, kind of services offered and requested by users, interference and noise between all elements of the network and several more have to be taken into account.
For data transmission, signals on a radio connection are modulated onto a carrier signal. This may be done by changing the phase, frequency, and/or amplitude of a carrier. Examples for digital modulation techniques are phase shift keying (PSK), frequency shift keying (FSK), or amplitude shift keying (ASK). In phase shift keying, the signal to be transmitted is modulated by changing the phase of a reference signal. Each one of a finite number of defined phases corresponds to a unique bit pattern, forming a symbol, which allows transmitting a digital signal of bits. A demodulator at the receiving end will then be able to extract the original signal from the detected phase or phase change. While any number of phases may be used for phase modulation, binary phase shift keying employing two phases and quadrature phase shift keying employing four phases are common examples. In a similar way, data is transmitted using frequency shift keying by changing the output frequency of the carrier signal, e.g. between two (binary FSK) or more discrete frequencies. Amplitude shift keying leaves frequency and phase of the carrier constant while changing the amplitude in order to transmit a signal, for example using two levels of amplitude representing a binary zero and one. More complicated modulation schemes are also known, such as quadrature amplitude modulation (QAM), where two out-of-phase carrier waves are amplitude-modulated. The term “quadrature” describes the 90 degree-phase shift between those carriers. Further techniques and combinations are conceivable. Furthermore, coding is used for adapting a signal to be transmitted. This may include improving transmission quality and fidelity, modifying the signal spectrum, increasing the information content, providing error detection and/or correction, and providing data security. A large number of coding schemes is known and readily used in the art, such as forward error correction.
Each modulation and coding scheme has its own strengths such as achievable bandwidth, and proneness to errors and interference may also vary with the selected scheme. The modulation and coding scheme used in a radio communication network thus has substantial influence on the achievable transmission rate. This fact is used in adaptive modulation and coding (AMC), also referred to as link adaptation. With AMC, the currently achieved signal quality and current channel conditions are used for determining the subsequently deployed modulation and coding scheme for transmitting data on a communication link. This may e.g. be achieved by feedback to the transmitter regarding the transmitted signal quality, or by assuming that the received signal quality is approximately that of the transmitted signal. While some coding schemes may support higher transmission rates or data throughput, others may e.g. be more robust and less sensitive to noise and errors at the expense of a lower bit rate. Schemes may be selected such that the signal-to-interference-and-noise ratio SINR and thus signal quality of the radio connection is optimized at any time. When the SINR falls below a predefined threshold value, the modulation scheme may be changed in order to achieve a better SINR. Further parameters of the connection link or the protocol used may be adapted along with the modulation and coding.
Another modulation scheme for data transmission, which may be applied in but is not limited to radio communication networks, is Orthogonal Frequency Division Multiplex (OFDM). OFDM is a modulation scheme based on multiple orthogonal sub-carriers. Each of the sub-carriers is modulated with a common modulation scheme such as those described above, e.g. QAM or PSK, at a low symbol rate. The orthogonality of the sub-carriers prevents cross-talk although the narrow frequency bands of the sub-carriers may be arranged very close together. The concept of OFDM may also be used for an access scheme, OFDMA (orthogonal frequency division multiple access). This basically means that different OFDM sub-carriers are assigned to different users. However, OFDM may also be combined with other access schemes such as time division (TDMA), frequency division (FDMA) or code division (CDMA) multiple access.
Example networks using OFDM/OFDMA are e.g. WiMAX (Worldwide Interoperability for Microwave Access), intended to provide wireless data transmission over long distances, or Flash-OFDM (Fast Low-latency Access with Seamless Handoff-OFDM) as a packet based mobile network. Both concepts and corresponding standards such as IEEE 802.16 for WiMAX are well known in the art, just as further networks applying OFDM, and will not be discussed in detail.
AMC as described above may be advantageously applied to OFDM systems, such that each of the orthogonal sub-carriers is subject to adaptive modulation and coding. This will further increase stability of the connection. Of course, AMC may alternatively also be applied across all or some sub-carriers simultaneously.
In a OFDM-based network, the number of users which may be served by one antenna will depend strongly on the interference caused by neighbouring antennas. When interference is high, the achievable signal-to-noise-ratio will be lowered, and thus a modulation scheme with less throughput but higher noise stability may be selected by the AMC. In turn, this interference from other antennas in the network is dependent on the location, settings, and load of these further antennas. The load of one of these antennas is again dependent on the number of users served by this antenna and the interference from all other antennas. As a result, transmission and user capacities of a single antenna cannot be considered separately, but has to factor in all antennas and users by coupled equations. Since the number of users and thus of user positions is in general substantially higher than the number of potential antenna positions, the presence of user positions within the relevant equations complicates the solution of such a system of coupled equations. As an example, 100 to 10000 antenna positions may have to be taken into account, but also up to 10 million user positions. This leads to tedious numerical calculations for evaluating a communication network.
In UMTS (Universal Mobile Telecommunications System) radio networks, the basic situation is similar. However, it is known that the adaptive power control used in UMTS networks may be linearized for purposes of such calculations. Together with averaging effects it is thus possible and common in the art to eliminate user positions from the coupled system of equations, which simplifies the remaining equations considerably. Only about 100 to 10000 equations usually remain, which can easily be solved by iterative numeric processes, thus allowing a simple and fast evaluation of UMTS network capacity.
The approach used for UMTS systems can not be transferred to OFDM based networks, as these do not use adaptive power control, but rather transmit all signals with equal power. Therefore, currently only time-consuming and complex simulations are feasible in order to evaluate the capacity of an OFDM-based radio network.